Those are actually one of the few things working correctly. I've been deleting things by making Azelstan my Prism Ranger Grenadier and I'm also in Chapter 4 pre HG and WORLD with the gun earrings and while im sure its a build that is by no means future proof it is absolutely tearing things in half before targets can even get in range or have any MP to fire back. Same goes for my Folcurt TK who has the G.Sword one and Canopus who has the dagger one, their damage is basically very beefy before Folcurt can inflict Breach or Fear.
It actually makes your DPS pretty consistent from what I can tell.
it's possible that you are right about them working correctly, but the truth is all they're really good for is defense against that weapon type
i did a simple little test (which i'd done before to confirm that the earrings were not worthwhile investments), the results of which i will paste here:
arycelle & lindl (both lightning element so not a factor), both fusiliers, same weapons/weapon skill/gear, lindl 12 dex higher
(ring of deftness +1 on both)
Lindl -> Arycelle: 311 damage
Arycelle -> Lindl: 288 damage
(gun earring on lindl)
Lindl -> Arycelle: 309 damage (-2)
Arycelle -> Lindl: 267 damage (-21)
(gun earring on both)
Lindl -> Arycelle: 287 damage (-22)
Arycelle -> Lindl: 252 damage (-15)
this is pretty trivial to reproduce with any other weapon type. as a consequence, the only really worthwhile (term used very loosely) earring is probably the cudgel one on casters who can afford to fit their weapon skill, since it will be a small reduction in the amount of aoe spell damage they eat from enemy mages
of course there's no real reason to not use them if you already have them, but given how late they come compared to the stat rings and how inconsistent the defense portion of weapon skill is, it's hard to come away with any assessment other than they are a disappointing acquisition
edit: hybrids may be something of an exception here since an earring will boost both physical and magical damage, though in practice this is pretty much limited to lord, princess, & valkyrie and lord isn't wasting a skill slot on weapon skill in the first place